1. Field of the Invention
The present invention generally relates to the field of electronics, and more specifically to an arrangement for preventing the temperature of an electronic assembly from rising above a safe value.
2. Description of the Related Art
Electronic components, such as integrated circuit chips, power transistors and the like will become damaged or destroyed if the temperature thereof is allowed to rise above a certain value, typically on the order of 200.degree. C. The normal operating temperature of an electronic assembly is generally maintained below this level by a heat sink arrangement, which may include convection air or liquid cooling. An alternative passive cooling arrangement involves the use of substrate materials such as beryllia (BeO) and aluminum nitride (AlN) which have high thermal conductivity and act as heat spreaders to conduct heat away from high power electronic components.
However, transient overload conditions often occur which cause the temperature of an electronic assembly to increase above the safe level for brief periods of time. The obvious expedient is to design the cooling system to accommodate the highest anticipated transient temperature. This is inefficient, however, since the size and capacity of the cooling system must be substantially larger than that required for normal operation.
Investigations involving solid solutions of polyhydric alcohols have been made for the purpose of passive heat storage such as described in U.S. Department of Energy report no. SERI/TP-255-2494 (DE85000535), entitled "SOLID STATE PHASE CHANGE MATERIALS FOR THERMAL ENERGY STORAGE IN PASSIVE SOLAR HEATED BUILDINGS", by D. Benson et al, Sept. 1984. These materials, known as phase change materials (PCMs), exhibit a reversible isothermal solid-solid phase transition at temperatures of 43.degree. C.-188.degree. C. During the transition, the crystalline structures of the PCMs change from layered to cubic in response to absorbed latent heat. For passive solar heating of buildings, the PCMs are used to absorb heat during the hot daylight hours and regenerate the heat during the cold nighttime hours. The latent heat (enthalpy) which is absorbed or regenerated during the phase transitions is high, on the order of 131-323 kilo-Joules per kilogram (kJ/kg), making polyhydric alcohols highly efficient for temporary heat storage as described in the report.